The efficiency with which the immune system cures or protects individuals from infectious disease has always been intriguing to scientists, as it has been believed that it might be possible to activate the immune system to combat other types of diseases. Such diseases include cancer, AIDS, hepatitis and infectious disease in immunosuppressed patients. While various procedures involving the use of antibodies have been applied in those types of diseases, few if any successful attempts using cytotoxic T cells have been recorded. Theoretically, cytotoxic T cells would be the preferable means of treating the types of disease noted above. However, no in vitro procedures have been available to specifically activate cytotoxic T cells.
Cytotoxic T cells, or CD8 cells as they are presently known, represent the main line of defense against viral infections. CD8 lymphocytes specifically recognize and kill cells which are infected by a virus. Thus, the cost of eliminating a viral infection is the accompanying loss of the infected cells. The T cell receptors on the surface of CD8 cells cannot recognize foreign antigens directly. In contrast to antibodies, antigen must first be presented to the receptors.
The presentation of antigen to T cells is accomplished by major histocompatibility complex (MHC) molecules of the Class I type. The major histocompatibility complex (MHC) refers to a large genetic locus encoding an extensive family of glycoproteins which play an important role in the immune response. The MHC genes, which are also referred to as the HLA (human leucocyte antigen) complex, are located on chromosome 6 in humans. The molecules encoded by MHC genes are present on cell surfaces and are largely responsible for recognition of tissue transplants as "non-self". Thus, membrane-bound MHC molecules are intimately involved in recognition of antigens by T cells.
MHC products are grouped into three major classes, referred to as I, II, and III. T cells that serve mainly as helper cells express CD4 and are primarily restricted by Class II molecules, whereas CD8-expressing cells, which mostly represent cytotoxic effector cells, interact with Class I molecules.
Class I molecules are membrane glycoproteins with the ability to bind peptides derived primarily from intracellular degradation of endogenous proteins. Complexes of MHC molecules with peptides derived from viral, bacterial and other foreign proteins comprise the ligand that triggers the antigen responsiveness of T cells. In contrast, complexes of MHC molecules with peptides derived from normal cellular products play a role in "teaching" the T cells to tolerate self-peptides, in the thymus. Class I molecules do not present entire, intact antigens; rather, they present peptide fragments thereof, "loaded" onto their "peptide binding groove".
For many years, immunologists have hoped to raise specific cytotoxic cells targeting viruses, retroviruses and cancer cells. While targeting against viral diseases in general may be accomplished in vivo by vaccination with live or attenuated vaccines, no similar success has been achieved with retroviruses or with cancer cells. Moreover, the vaccine approach has not had the desired efficacy in immunosuppressed patients. One way around this difficulty would be to immunize a heathy individual, isolate the CD8 cells from this individual, and inject these CD8 cells into the disease-afflicted person. This experimental protocol seems to work in inbred mouse strains, but it has not been successfully tried in humans. There are several possible explanations. First of all, peptides are unique for a given MHC; in other words, certain antigenic peptides bind preferentially to particular MHC species and do not bind well to others, even in the absence of the "preferred" MHC molecule. Furthermore, MHC molecules are highly polymorphic, which fact generates at least two problems. First, the CD8 cells of an individual can only interact with peptides bound to precisely those three to six Class I molecules present in that individual. Second, CD8 cells react violently with all Class I molecules which are different from those expressed in the individual from whom the CD8 cells are obtained, regardless of what peptides the Class I molecules contain. This reactivity has been observed for some time and is termed allo-reactivity. It is the underlying cause of the immune rejection of transplanted organs.
Thus, apart from the rather heroic experimental protocol in which one individual is used as the donor of activated CD8 cells to another individual, it is difficult to find two unrelated persons with the exact same setup of Class I molecules. For this reason, at least one researcher has taken the rather non-specific approach of "boosting" existing CD8 cells by incubating them in vitro with IL-2, a growth factor for T cells. However, this protocol (known as LAK cell therapy) will only allow the expansion of those CD8 cells which are already activated. As the immune system is always active for one reason or another, most of the IL-2 stimulated cells will be irrelevant for the purpose of combatting the disease. In fact, it has not been documented that this type of therapy activates any cells with the desired specificity. Thus, the benefits of LAK cell therapy are controversial at best, and the side effects are typically so severe that many studies have been discontinued.
Several novel molecules which appear to be involved in the peptide loading process have recently been identified. It has also been noted that Class I molecules without bound peptide (i.e., "empty" molecules) can be produced under certain restrictive circumstances. These "empty" molecules are often unable to reach the cell surface, however, as Class I molecules without bound peptide are very thermolabile. Thus, the "empty" Class I molecules disassemble during their transport from the interior of the cell to the cell surface. This is an elegant means by which the immune system can ensure that only cells that are actively synthesizing viral proteins are destroyed. For example, when a virally infected cell is killed, it will release viral peptides. If neighboring cells were expressing "empty" Class I molecules--i.e., those without bound peptide--these cells would be coated with the released viral peptides. Since cytotoxic T cells (or CD8 cells) have no means of ascertaining how or why a peptide happens to be bound to a Class I molecule, cells which passively obtained a viral peptide coating would be killed as well as those which were actively synthesizing the viral proteins.
Viral peptides in vivo are broken down by a large particle known as the proteasome. This enzyme complex also breaks down normal, cellular proteins, which suggests that peptides derived from our own cellular proteins compete with virally-derived peptides for binding sites on Class I molecules. Thus, only some of the Class I molecules on the surface of a virally-infected cell would actually contain viral peptides, as the majority of the Class I molecules would contain peptides derived from our own, cellular proteins. As there are tens of thousands of Class I molecules on the surface of a cell, and as CD8 cells can recognize as few as 200 Class I molecules loaded with a given viral peptide, this competition between peptides derived from viruses and cellular proteins does not completely compromise the efficiency by which the CD8 cells can destroy a virally-infected cell. Nevertheless, if one were able to "engineer" cells to express Class I molecules displaying only one species of antigenic peptide--i.e., if all Class I molecules had the same peptide bound thereto--it would arguably increase the efficiency of CD8 activation.
Class I molecules bind peptides in a specific manner. All peptides have to be about 8-9 amino acids in length and their sequences must fit the peptide-binding pocket of the Class I molecules. In this respect, Class I molecules display some resemblance to antibodies. However, while a given antibody tends to bind only one antigen, a given Class I molecule can bind many hundred different peptides. As the number of viruses and other pathogens is quite large, it is apparent that our immune defense would be poor if we had only a single Class I molecule, even if it is capable of binding and altering many different peptides. For this reason, all humans have between three and six different Class I molecules, which can each bind many different types of peptides. Accordingly, the CD8 cells can recognize many thousands of peptides bound to one or another Class I molecule.
As selection seems to be the dominant force in evolution, pathogens emerge which cannot be recognized efficiently by the immune system. Thus, a viral sequence, which gives rise to peptides that bind efficiently to a variety of Class I molecules, may mutate such that it is not recognized by any of the three to six Class I molecules present in an individual. This virus may therefore not be recognized by the immune system and may consequently cause the death of the affected individual. If all individuals had an identical set of Class I molecules, such a virus might conceivably eliminate an entire species. However, individual variation is a safeguard against that possibility, as some 100 different forms of Class I molecules are present in the population. Thus, a mutated virus may injure some individuals, but it is unlikely to result in the extinction of an entire population, since different individuals possess different sets of Class I molecules.
If Class I molecules can bind a variety of peptides, including peptides derived from our own cellular proteins, one may wonder why the CD8 cells of the immune system do not recognize and destroy our own tissues. While the answer to this question is not entirely clear, two distinct mechanisms are presently believed to be operating. First, CD8 cells that can react with self-peptides are eliminated in the thymus. Second, CD8 cells become non-responsive (anergic) to self-peptides in the peripheral organs of the immune system. Since every possible type or epitope of cellular proteins is not synthesized by the cells in the thymus, the second mechanism would appear to be the more likely explanation. This mechanism appears to be operational for the level of self-peptides normally encountered. If this level is increased by some means, it can be shown that individuals do indeed have CD8 cells that can recognize and destroy cells expressing self-peptides. This latter observation is significant with regard to the concept of using the immune system to eliminate tumor cells.